1. Field of the Invention
The present invention relates to a process for producing high purity methoxyanthraquinones. More particularly, the invention relates to an improvement in the process for producing methoxyanthraquinones by reacting nitroanthraquinones with a methanol-alkali mixture.
2. Description of the Prior Art
The production of methoxyanthraquinones from nitroanthraquinones as a starting material is well known as disclosed, for instance, in German Pat. No. 77,818. This disclosure shows a process in which a mixture of dinitroanthraquinones, obtained by dinitrating anthraquinones with a mixed acid solution of concentrated sulfuric acid and nitric acid, is reacted with an alkali metal or an alkaline earth metal hydroxide in a methanolic solution. However, as mentioned in German Offenlegungsschrift No. 2,152,991 (Japanese Patent Provisional Publication No. 51017/1973), this particular process produces about 30-40% by weight of by-products such as diamino-, aminomethoxy-, hydroxymethoxy-, dihydroxy- and aminoanthraquinones. The disclosure of the German Offenlegungsschrift, in turn, also discloses a process for producing 1,5- or 1,8-dimethoxyanthraquinone by heating 1,5- or 1,8-dinitroanthraquinone with potassium hydroxide in methanol. Even when the reaction is conducted in this manner, it is still difficult to control the side reactions which occur and consequently, dimethoxyanthraquinones are produced which contain 10-20% by weight of by-products.
In another prior art approach to the reaction, German Offenlegungsschrift No. 2,314,696 (Japanese Patent Provisional Publication No. 126660/1974) discloses the production of a .alpha.-methoxyanthraquinones using potassium carbonate anhydride as the alkali agent. An example of the patent publication shows that the nitrogen content in the .alpha.-methoxyanthraquinones produced is less than 0.5%, which is equivalent to about 10% of by-products, calculated on the assumption that the nitrogen comes from the main by-product, aminomethoxyanthraquinone.
A need therefore, continues to exist for a method by which methoxyanthraquinones can be produced from nitroanthraquinones containing substantially reduced amounts of by-products.
In the extensive research which has led to the present discovery, it has been found that the presence of dissolved oxygen in the reaction mixture has a substantial influence on the side reactions which occur, and that when the reaction is conducted in the absence of dissolved oxygen, a large amount of by-products are produced.